Top ten challenges to Femtocell deployment--Part II

6. Quality of Service and Traffic Prioritization
Due to a stronger RF signal based on better indoor coverage, femtocells will provide faster handset data speeds than what mobile subscribers may be used to, which is expected to encourage greater use of data services and video streaming inside the home. This is probably the most important strategic reason for a mobile operator to deploy femtocells in its network, as femtocells could enable the mobile phone to compete not only with the fixed line phone, but also with the PC and TV for entertainment and information services in the home. Voice is real-time and latency-sensitive, which means that best-effort IP delivery is not good enough. Delivering high quality of service is a major challenge when there is a shared IP access link simultaneously carrying voice, video, peer-to-peer (P2P) traffic, and data. In this operating environment, traffic prioritization becomes essential--not only for effective handling of real-time voice packets, but also for prioritizing network time protocol (NTP) or IEEE 1588 timing packets. If timing packets are not prioritized, then femtocells can go out-of-sync and lead to dropped calls, which results in poor subscriber satisfaction. DSL uplink bandwidth, in certain deployments, adds additional constraints. In the U.S., for example, depending on the distance from the CO, many mid-tier plans have only 200K throughput on uplink. With other devices like PCs that have active VPN connections sharing the same uplink with the femtocell equipment, traffic prioritization becomes essential to ensure consumer experience does not deteriorate with events like VPN connection breaking when making a mobile voice call.

7. Timing and Network Synchronization
Attaining required timing and network synchronization levels is a major femtocell challenge. 3GPP specifies that base station transmit frequencies be very accurate and closely synchronized, requiring precise clock references which add to base station costs. The standard has been relaxed for picocells in Release 6, and it has been proposed that future versions of the standard relax it even further. Lower-cost synchronization solutions are still needed.

To meet stringent network synchronization requirements, femtocells can use IEEE 1588 for an accurate clock synchronization protocol between networked equipment. As picoChip points out, "[B]y distributing a high precision time base around the network that is resilient to the typical levels of packet delay and jitter found on broadband Internet connections, a low-cost implementation is possible." Alternatively, a GPS timing reference could be used, or the base station could receive transmissions from the overlaying macro cellular network and adjust its timing accordingly. Finally, there are some innovative, low-cost / high-stability temperature-controlled crystal oscillators coming onto the market that may make it easier to attain the required synchronization levels. More study is needed for this critical facet of femtocell operation.